Physical Sciences Division Research Highlights

September 2013

New Technique Efficiently Resolves Chemistry of Nanoparticles

Elements clearly identified on 3D map

A new technique combining scanning transmission electron microscopy and X-ray energy dispersive spectrometry showed this 3D nanoparticle used as a cathode material in lithium ion batteries. The technique enables clearer visualizations at much faster rates than currently used methods. Enlarge Image

Results: A new
technique from Pacific Northwest National Laboratory and FEI Company lets scientists
efficiently resolve elements' locations in three dimensions. The team's technique
combines scanning transmission electron microscopy and X-ray energy dispersive
spectrometry with a new detector arrangement and a brighter electron beam. The result
is a three-dimensional map of the elements' placement on a sample smaller than
a single blood cell. The team applied this technique to a lithium-rich nickel-based
material that could be part of tomorrow's batteries. They discovered how nickel
was segregating away from other elements on the material's surface.

"This technique gave us our fastest,
cleanest view yet," said Dr. Chongmin Wang, a materials scientist with the
national laboratory's Chemical Imaging Initiative. "The paper is proving
popular; it is Ultramicroscopy's most
downloaded article in the last 90 days."

Why It Matters:
Scientists, along with the rest of the population, want answers quickly and
accurately so they can focus on what matters most. The team's technique provides
precise 3D chemical images in hours, not days, and avoids the time and expense
of reshaping samples and transporting them to other instruments. The
information generated by this technique could help in the intentional, versus
trial and error, material design of longer lasting, higher capacity batteries.

For the spectrometry, the team
arranged four windowless silicon drift detectors around the sample. The
detectors, with improved tilt response, quickly scanned the sample. Because the
electron beam did not stay on a single spot for more than 25 microseconds, the
scientists avoided "parking" issues, where the electron beam lingers in a
single spot and damages the sample. The scans from the four detectors were
combined and joined with the microscopy information using specialized software.

This technique is faster and
provides a wider field of view than more traditional 3D techniques such as scanning
electron microscopy combined with electron energy-loss spectrometry or atom
probe tomography. In 3 hours, the team obtained data sets from 29 microscopy
images and elemental maps. Other techniques can take up to a day and do not
provide as clear an image. Further, this single instrument provides a wider
field than similar chemical techniques and allows scientists to see individual particles
without additional preparation that could modify the native structure.

"It is now possible to obtain 3D composition
maps from nanoparticles in their native state and reduce the total time to reconstruct
chemical information," said Dr. Libor Kovarik, a PNNL scientist on the team.

What's Next? The team continues
to investigate how elements aggregate and drift in lithium-ion batteries and
other energy storage materials. In addition, they are refining their chemical
imaging techniques, striving to provide better ways to gather detailed
information.